1. Technical Field
The present invention relates to a method of producing a metallic backing material provided with an overlay, having the features of the precharacterizing clause of claim 1, together with the use of such coated backing materials.
2. Related Art
Metallic backing materials provided with a plastics overlay are known which may be used to produce sliding or bearing elements, such as bushes, half-shells, sliding rails or sliding plates, whether unlubricated or with additional oil or grease lubrication.
As a rule, composite materials with a plastics overlay are used for such sliding elements, which materials consist of a metallic backing framework, preferably of steel, bronze or a high-strength aluminum alloy, which takes the form of a sintered porous surface layer or of a fabric, and a continuous plastics layer applied to the metal framework in such a way that the spaces in the framework structure are completely filled with the plastics materials. Materials without a framework structure are also known, in which a plastics film is adhered to a smooth backing material.
In principle, there are two significant embodiments of sliding materials, which differ from one another in the matrix plastics used and the associated different modes of operation. The materials are based on the one hand on polytetrafluoroethylene and on the other hand on thermoplastics.
Thermoplastic-based materials, such as for example POM, PVDF or PA, belong to the prior art. These operate differently from polytetrafluoroethylene materials. In the case of thermoplastic-based materials, the layer thickness is generally greater than 50 μm, wherein the metallic backing framework is not involved in the sliding process but merely serves as a mechanical anchor for the thermoplastic. However, such materials have different properties from polytetrafluoroethylene materials, both from a positive and from a negative point of view. For instance, these thermoplastic materials have higher coefficients of friction in the case of oil lubrication, which is a particular disadvantage for hydraulic applications, and poorer plasticity, whereby for example it is difficult or impossible to produce flanged bushes or bushes without metal backing, while post-machining, for example drilling in the installed state to compensate housing tolerances, is possible, together with a relatively high load-carrying capacity and wear resistance.
Polytetrafluoroethylene-based materials are substantially softer and their functionality is therefore generally based, in the absence of lubricants and under appreciable load, for instance greater than 1 MPa, on the exposure of the metallic backing framework, such that the latter is involved in frictional contact as a surface component. In the case of such materials, when conventional plastics compositions are used it is not sensible to build up layers over 50 μm thick, since these layers cannot absorb the load without flow. On the contrary, they generally have relatively small layer thicknesses of around 20 μm, which contain precisely enough material for forming a transfer layer on the counterpart. Surplus material leads to the formation of undesired wear particles during the running-in phase and may result in undesired material displacement in the case of the generally necessary calibration processes during manufacture or during installation of the sliding elements.
These materials may be produced relatively simply, starting with a polytetrafluoroethylene dispersion with which appropriate additions are mixed and which is then converted into a homogeneous plastic blend by coagulation. The paste produced in this way is then applied by means of rolls to the strip-form preliminary material with the framework structure and finally sintered, as described for example in DE 195 06 684, a layer thickness of 0-50 μm also being indicated therein.
Materials are described in DE 198 08 540 A with which the overlay may consist of polytetrafluoroethylene with 10-50% of poly(p-phenylene terephthalamide) (PPTA), wherein the layer thicknesses of these materials are limited to 0-50 μm by rolling into the porous substrate. Toluene is used for production.
Thick polytetrafluoroethylene-based layers may also be produced, but using a comparatively expensive and complicated method. This known method firstly requires the production of a film of the respective material, which has then to be applied to the backing material. In comparison to the layers of the same composition produced by the paste method, these films generally exhibit better mechanical properties and higher wear resistance, since they are peeled off blocks which have been sintered under more effective conditions than is the case when sintering overlays in a tunnel oven. The films may be stuck on as a smooth backing material or applied without adhesive to a heated, porous backing framework.
However, it is deemed a disadvantage that first of all it is always necessary to perform additional operations, namely production of the sintered block and peeling of the film. If it is also necessary to use an adhesive, the film has additionally to be etched with a solution of sodium in ammonia or with sodium naphthyl, which is problematic from an environmental and safety point of view. Sticking the film on by means of an adhesive is likewise problematic, since fluorinated high temperature adhesives are used, wherein heating and cooling have to be performed while the pressing pressure is maintained. In any case, adhesive strength is limited, such that the layer may easily become detached especially at cut edges.
Additional prior art which should be mentioned is DE 44 30 474, which discloses a method in which a film consisting mainly of thermoplastics is incorporated into a porous substrate, and EP 0 877 867, which describes the production of a thermoplastic layer by sprinkling on a powder, followed by melting and rolling smooth. Thermoplastic layers have hitherto had a greater load-carrying capacity then polytetrafluoroethylene layers and may therefore be thicker and, if necessary, also post-machined. U.S. Pat. No. 5,019,308 proposes a method of producing relatively thick layers, in which a blend of polytetrafluoroethylene and fillers is firstly applied to a plastics film and pre-dried, in order then to be transferred in a next method step to the metallic substrate material proper. However, this method is involved and thus cost-intensive, since it requires an additional process step and, moreover, the indicated compositions do not produce strong layers.